Japanese Laid-open Patent Publication No. Hei. 11-166600, U.S. Pat. No. 6,142,902, and Japanese Utility Model Publication No. 2553206, describe double-sided silent chains having teeth protruding both toward the inside and toward the outside of a loop. Sprockets engaged with the outwardly protruding teeth and sprockets engaged with the inwardly protruding teeth rotate in opposite directions.
Double-sided silent chains have been used as timing chains in automobile engines. In the case of a timing chain, the inwardly protruding teeth are engaged with a crankshaft sprocket, and one or more camshaft sprockets, for example, sprockets on an intake valve cam and an exhaust valve cam respectively, so that the crankshaft and camshaft sprockets rotate in the same direction. The outwardly protruding teeth may be engaged with sprockets for driving auxiliary equipment such as an oil pump and the like. The sprockets driving the auxiliary equipment rotate in a direction opposite to the direction of rotation of the crankshaft and camshaft sprockets.
FIGS. 7 and 8 show an example of a conventional double-sided silent chain 51, composed of link plates 55 and 56, each having a pair of teeth 52 and a pair of pin holes 53. The plates are interleaved, alternately displaced longitudinally, and connected in an endless loop by connecting pins 57. Link plates 56 extend in two side-by-side, lengthwise columns one being on the left side of the chain and the other being on the right side of the chain. The teeth 52 of links 56, protrude upward in FIG. 8. The teeth 52 of the intermediate links 55 protrude downward. FIGS. 7 and 8 show an upper run of the chain. The upwardly protruding teeth protrude toward the outside of the loop, and are adapted to drive sprockets on auxiliary equipment, and the downwardly protruding teeth protrude toward the inside of the loop and are adapted to be engaged with a crankshaft sprocket and one or more camshaft sprockets. The link plates 55 are arranged so that their back surfaces 54 are disposed in a common plane when the chain is straight. Likewise, link plates 56 are arranged so that their back surfaces 54 are disposed in a common plane when the chain is straight. The connecting pins 57 are fitted to the pin holes 53 of the outermost link plates 56, and fixed only to the outermost link plates, thereby allowing articulation of the widthwise rows of link plates.
FIGS. 9 and 10 each show a lower run of the same chain in meshing engagement with an external sprocket 58, i.e., a sprocket outside the loop of the chain, on a shaft 60. The sprocket, the structure of which can be understood more clearly from FIG. 12, has two, axially spaced, sets of teeth 59. In FIG. 9, the section plane intersects the middles of two of the outermost link plates 56, whereas in FIG. 10, the section plane intersects the middles of two link plates 56 adjacent the outermost link plates. FIG. 9 shows a location at which sprocket teeth 59 are in engagement with outer tooth surfaces of the link plates 56 located adjacent the outermost link plates. FIG. 10 shows a location at which the sprocket teeth 59 are in engagement with the outer tooth surfaces of the outermost link plates. As shown in FIG. 9, the outermost plates of the set of inner link plates 55 come into sliding contact with the inner side walls of the sprocket teeth 59, to prevent lateral vibration of the chain.
FIG. 11 shows the engagement of link teeth 52 of inner link plates 55, which protrude toward the inside of the chain loop, with teeth 62 of a conventional sprocket 61 on a shaft 63
In the conventional double-sided silent chain 51, as described above, tension is generated in the inner circumferential portions of the pin holes 53 of the outermost link plates when the connecting pins 57 are fitted into the pin holes 53. Collision of the teeth of the outer link plate with the teeth of a driven sprocket external to the chain loop also applies a stress to the inner circumferential portion of the pin holes. The tension and stress produce a synergistic effect, lowering the strength of the outermost link plates.
The inner link plates 55 and the outer link plates 56 of a conventional double-sided silent chain typically have the same shape. Since the minimum distance between the back of a link plate and its tooth gap bottom (distance H in FIG. 5) is comparatively low, the middle portions of the link plates 55 and 56 are relatively weak. This weakness, in the outermost link plates, manifests itself in the formation of cracks near the tooth gap bottom, a progressive decrease in the strength of the outermost link plates, and a reduction in rotational fatigue strength of the chain.
The problems caused by weakness in the outermost link plates of a double-sided silent chain can be addressed by substitution of guide plates having no teeth for the outermost, toothed, link plates. However, with this approach, the teeth of the link plates adjacent the guide plates are easily worn. To suppress wear of the teeth of these link plates, their thickness can be increased, or additional link plates can be provided. However increasing the width of a link plate, or adding link plates, results in a disadvantageous increase in the width of the chain.
The principal objects of this invention are to solve the above-mentioned problems, and to provide a double-sided silent chain in which the decrease in strength at the central portions of the outermost link plates is prevented, and rotational fatigue strength of the chain is improved.